The present invention relates to an electrophotographic transfer sheet used in an apparatus for forming a color image, such as a color duplicator, a color printer and a color facsimile machine, to which an electrophotographic process is applied, and to a process for forming a color image using the transfer sheet, and more particularly, the invention relates to an electrophotographic transfer sheet that relief on an image transferred to the electrophotographic transfer sheet are lightened, the gloss property is increased to improve the image quality, and cracks formed on bending the electrophotographic transfer sheet are suppressed, and to a process for forming a color image using the transfer sheet.
One example of the apparatus for forming a color image, such as a color duplicator and a color printer, to which an electrophotographic process is applied, has only one photoreceptor drum, is equipped and is constituted in such a manner that toner images of respective colors, yellow (Y), magenta (M), cyan (C) and black (BK), are consecutively formed on the photoreceptor drum, and after transferring and accumulating the toner images of respective colors, yellow (Y), magenta (M), cyan (C) and black (BK), consecutively formed on the photoreceptor drum on a transfer sheet, the toner images are heated to be fixed on the transfer sheet, so as to form a color image. Another example thereof is constituted in such a manner that the toner images of respective colors, yellow (Y), magenta (M), cyan (C) and black (BK), consecutively formed on the photoreceptor drum are once primarily transferred and accumulated on an intermediate transfer material, and the toner images of the respective colors are then secondarily transferred to a transfer sheet all at once, followed by heating the toner images to be fixed on the transfer sheet, so as to form a color image.
A further example thereof has plural image forming units corresponding to respective colors, yellow (Y), magenta (M), cyan (C) and black (BK), and is constituted in such a manner that toner images of respective colors, yellow (Y), magenta H), cyan (C) and black (BK), consecutively formed on photoreceptor drums of the image forming units are transferred and accumulated on a transfer sheet, or in alternative, they are once primarily transferred and accumulated on an intermediate transfer material, and the toner images are then secondarily transferred to a transfer sheet all at once, followed by heating the toner images to be fixed on the transfer sheet, so as to form a color image.
The color toner transferred and fixed on the transfer sheet is generally constituted by dispersing or melting and mixing a coloring agent formed from a pigment or a dye in a binder resin or by polymerizing a monomer with a dye and wax in an aqueous system, and the particle diameter thereof is generally set at from several micrometers to several tens micrometers. The color toner is transferred to ordinary paper or coated paper, in such a state that plural toner layers are accumulated, and the color toner is fixed by heating and melting on ordinary paper or coated paper, whereby a color image is formed. At this time, unevenness of, for example, about from 10 to 100 xcexcm is formed on the surface of the color image due to ups and downs of the toner layers to form gloss nonuniformity. As a result, the color image formed on ordinary paper or coated paper, irregularly reflects incident illumination light and appears as an image with poor gloss uniformity upon observing by the naked eye.
JP-A-5-216322 discloses a process, in which a transparent resin layer is set up on the surface of the transfer sheet, and a color toner is fixed on the transparent resin layer to form a color image, by which a color image excellent in gloss with rich color tones, excellent color reproducibility and high resolution can be obtained.
However, in the technique disclosed in JP-A-5-216322, in which a transparent resin layer is set upon the surface of the transfer sheet, and a color toner is fixed on the transparent resin layer to form a color image, when the transfer sheet 100 is bent as shown in FIG. 10, stress is concentrated at a part of poor strength of the transparent resin layer 101 to form a crack at that part, so as to deteriorate the image quality.
JP-A-6-19178 discloses a color image transfer sheet and a process for forming a color image, by which a color image having good gloss and good preservation property with high quality and no crack can be provided by the electrophotographic process.
JP-A-11-231562 discloses a process for forming an image, by which an image excellent in gloss uniformity and color reproducibility with good graininess can be obtained, and a crack on the surface layer of a recording sheet after forming an image and blocking defect does not occur.
The color image transfer material disclosed in JP-A-6-19178 is an image transfer material for forming a color image with three or more kinds of color toners that is constituted to have a transparent resin layer formed with a styrene-acrylic series resin containing a styrene-butadiene copolymer on the surface of the image transfer sheet.
The process for forming a color image disclosed in JP-A-6-19178 is a process for forming a color image by melting a toner image formed with three or more kinds of color toners and fixing it on an image transfer sheet, in which a transparent resin layer formed with a styrene-acrylic series resin containing a styrene-butadiene copolymer is provided on the surface of the image transfer sheet, and the toner image is melted and fixed on the image transfer sheet with a transporting and heating material of a belt form.
The process for forming an image disclosed in JP-A-11-231562 is a process for forming an image containing a step of transferring and fixing a toner image that has been transported to the prescribed toner image transferring position with a toner image retainer, which retains the toner image and transfers the toner image from the prescribed toner image forming position to the toner image transferring position, on the prescribed recording sheet by heating at least one of the toner image retainer and the recording sheet. In the process, the recording sheet is constituted in such a manner that the recording sheet has a thermoplastic transparent resin layer formed from two layers, i.e., an undercoating layer and an overcoating layer, at least on the surface of a substrate, on which the toner image is to be transferred, in which the softening point (Tm2) of the undercoating layer of the transparent resin layer and the softening point (Tm1) of the toner have the relationship, Tm2 less than (Tm1xe2x88x9220)xc2x0 C., and the softening point (Tm3) of the overcoating layer of the transparent resin layer and the softening point (Tm1) of the toner have the relationship, Tm1 less than Tm3 less than (Tm1+20)xc2x0 C.
In the process for forming an image disclosed in JP-A-11-231562, the recording sheet is constituted in such a manner that the glass transition temperature (Tg1) of the undercoating layer of the transparent resin layer satisfies the relationship, Tg1 less than 50xc2x0 C., and the glass transition temperature (Tg2) of the overcoating layer of the transparent resin layer satisfies the relationship, Tg2 greater than 60xc2x0 C.
However, the foregoing conventional techniques involve the following problems. That is, the color image transfer material of JP-A-6-19178 shown in FIG. 12 has such a problem that, in the image transfer sheet 200 for forming a color image with three or more kinds of color toners, which has, on the surface thereof, a transparent resin layer 201 formed with a styrene-acrylic series resin containing a styrene-butadiene copolymer, when a color image is fixed on the image transfer material 200, a color image cannot be buried on the transparent resin layer 201 depending on the properties of the transparent resin layer 201, and therefore relief are formed on the color image formed on the image transfer material 200 to fail to provide sufficient gloss uniformity, It has another problem in that, depending on the addition amount of the styrene-butadiene copolymer contained in the transparent resin layer 201, formation of cracks cannot be certainly prevented.
In the case of the process for forming an image disclosed in JP-A-11-231562 as shown in FIG. 13, the recording sheet 300 has, at least on the surface of a substrate 301, on which a toner image is to be formed, a thermoplastic transparent resin layer 304 formed with two layers, i.e., an undercoating layer 302 and an overcoating layer 303, in which the relationships among the softening point (Tm2) of the undercoating layer 302 of the transparent resin layer 304, the softening point (Tm3) of the overcoating layer 303 of the transparent resin layer 304 and the softening point (Tm1) of the toner are specified, and the glass transition temperature (Tg1) of the undercoating layer 302 of the transparent resin layer 304 and the glass transition temperature (Tg2) of the overcoating layer 303 of the transparent resin layer 304 satisfy the relationships, Tg1 less than 50xc2x0 C. and Tg2 greater than 60xc2x0 C. However, this technique involves such a problem that since the thermoplastic transparent resin layer 304 has the two-layer structure, the overcoating layer 303 cannot follow thermal expansion of the undercoating layer 302 when the thermal expansion coefficient of the undercoating layer 302 is larger, and therefore, cracks are formed in the case where the recording sheet 300 is placed in a high temperature environment.
The invention has been made in view of the foregoing circumstances and is to provide an electrophotographic transfer sheet and a process for forming a color image using the same, in which formation of relief of a toner image on the surface of the electrophotographic transfer sheet can be prevented to obtain sufficient high gloss and gloss uniformity, and formation of cracks on the surface of the electrophotographic transfer sheet can be prevented.
According to an aspect of the invention, the electrophotographic transfer sheet contains a substrate having one surface thereof an image receiving layer containing a thermoplastic resin as a main component, a resin constituting the image receiving layer having a melt viscosity at 120xc2x0 C. of about from 200 to 2,000 Paxc2x7s and an elongation after fracture of about 50% or more.
According to another aspect of the invention, the process for forming a color image contains the steps of: transferring a toner image containing a color toner on the image receiving layer of the electrophotographic transfer sheet of the invention; and fixing the toner image containing a color toner transferred to the image receiving layer of the electrophotographic transfer sheet by a belt type fixing unit to form a color image, the belt type fixing unit containing a fixing belt supported as being capable of circulating by plural rolls containing a heating roll, a pressure roll being in contact under pressure with the heating roll through the fixing belt, the electrophotographic transfer sheet being passed through the contact part under pressure between the fixing belt and the pressure roll to make the toner image face the fixing belt, so as to fix the toner image by heat and pressure, and the electrophotographic transfer sheet being released from the fixing belt under a condition where the fixing belt is cooled to a certain extent.